1. Field of the Invention
The invention is in the field of medical imaging and more specifically in the field of computerized tomography.
2. Related Art
Computerized tomography (CT) is an imaging technique wherein x-rays are used to obtain two-dimensional projection images at a variety of different angles around a target being examined. Computer techniques are then used to generate a three-dimensional representation of the target by combining the two-dimensional projection images. The three-dimensional representation can be viewed, sliced and rotated by a user.
CT systems can generally be characterized by the energies of the x-rays used, such as kilovoltage (kV) and megavoltage (MV) imaging. In kV imaging, x-rays with energies in the kiloelectronvolt range are generated and detected. In MV imaging, x-rays with energies in the megaelectronvolt range are generated and detected. Each of these types of imaging has advantages and disadvantages. For example, kV imaging may be subject to interference from tooth fillings and MV imaging may cause radiation damage to the DNA of living cells. MV imaging is sometimes used therapeutically as a cancer treatment.
In diagnostic CT imaging hundreds of two-dimensional projection images are recorded as an x-ray source and detector are rotated around the target. The quality of the final three-dimensional representation is dependent on the number of two-dimensional projection images used to generate the three-dimensional representation. The time required to record hundreds of two-dimensional projection images can be a problem when the target is a patient because the patient must stay still during the imaging process. Typically, diagnostic CT imaging is performed using kV imaging because of the danger to the patient of using MV x-rays to generate so many projection images.
One therapeutic use of MV x-rays is referred to as intensity-modulated radiation therapy (IMRT). IMRT enables caregivers to deliver an extremely conformal dose of high energy x-rays to a well defined treatment volume while minimizing radiation damage to nearby organs and tissues. The success of IMRT is largely dependent on the accuracy of patient positioning and target localization. Therefore, it is important to have an efficient and effective method to confirm the position of the patient and the target volume within the patient. Without confirmation of the position of the target volume, the x-ray dose may harm healthy tissue and miss the tissue requiring treatment. In many situations a volume that is larger than the volume of tissue to be treated is exposed to high energy x-rays in order to compensate for errors in patient positioning, organ motion, and target localization uncertainties. This results in an undesirable exposure of healthy tissue to these x-rays.
There is, therefore, a need for improved methods of imaging that provide greater speed of analysis and greater accuracy for target localization.